‘I’m about to open the result,” Sue Mitchell announces excitedly when she answers her phone. “Oh, wow,” I reply, knowing it’s a result that could change not just her life, but the dynamic of her whole family. “I’ll leave you to it. We can talk later.”

Do-it-yourself DNA tests have never been so popular. While a decade ago, a test would have set you back more than £300, and the results would have taken at least a week, you can now pick one up from a high street store such as Home Bargains for less than £100 and get the result the same day.

Today is that day for Sue. Ever since she was 20 – she’s 59 now – she has increasingly wondered if the man who brought her up is really her father. “I’m one of six siblings and while they are all tall and blond, I’m dark and short. They are clever, academic types, whereas I’m not. I have a completely different blood type too,” she’d told me earlier that day.

Sue’s mother is dead and although her dad is still alive, he’s 90 and is showing signs of dementia. “So I asked my sister to do a DNA test to see if we are full siblings,” says Sue, who adds that if the result is negative, she has a hunch about who her father might be. “There was a family friend, a dark looking man, who used to come to the house and would always single me out of the six of us, giving me by far the most attention. It makes you wonder.”

Sue’s story is not unusual, says David Thomas, director of Alphabiolabs, the biggest DNA testing laboratory in the UK and the fourth biggest in Europe. The lab, which started up at Liverpool University in 2004, now employs 39 people and is based in a Warrington business park. Downstairs, which looks like any other office with its clusters of desks and staff chatting away on headsets, is home to the business and customer service side of things; upstairs scientists in white coats process the DNA samples using huge, complex machinery at an impressive speed.

The day can start as early as 3.15am, says the head scientist, especially on Wednesdays on Thursdays, when the Jeremy Kyle Show – one of its regular clients – send in DNA tests, the results of which are to be revealed on that day’s show.

So how does Alphabiolabs deal with direct inquiries from the public? “I’m not saying we’re Samaritans, but we don’t let anyone near the customer service phone until they have had a month’s training and that includes counselling and signposting skills,” says Thomas. “These tests can change the lives of individuals and their families instantly and dramatically – we don’t underestimate the power of that.”

Ashley Hodgkinson, who is on the phones most days, recalls one woman who insisted her test result would mean she’d be sent back to Pakistan, where she might potentially be stoned to death. “It was a heart-wrenching story and we had many desperate calls from her. She’d had a relationship outside her marriage and the paternity test of her baby had given it away. She begged us to lie about the results but, of course, we couldn’t.”

People can feel anxious when they are waiting for the results, says Hodgkinson. “Often, people have been planning the test for years, but once they have sent in the swabs, they get very nervous, often phoning up several times to ask if it’s ready.”

Mandy Williams knows how they feel. “I’ve been feeling very tense today,” she says. Like Sue, she is having a sibling test. A friend, who happens to be a medium, told her over a lunch out a few years ago that her late father was not in fact her dad.

“I didn’t really know what to make of it, but she seemed so sure,” says Mandy. “Eventually, I decided to go to another medium and if she mentioned it, I’d do a test. The other medium did, so here I am. I must admit it has freaked me out as I’d never had any doubts before. My sister reluctantly agreed to be tested with me and if it’s negative, I’ll be confronting my mum this weekend,” she says.

We agree to talk again when she has the results. “We’re full siblings,” she proclaims immediately, clearly relieved. “I’m not sure how I feel about my friend at the moment, though.”

Around 15% of test results are inconclusive, admits Thomas, although that’s more to do with the chromosomal make-up of the sample than the testing process. “But in those cases, we try to help find another way.”

One 33-year-old customer explains how he first used Alphabiolabs late last year, when he discovered that his dad wasn’t his biological father. “I found my potential biological father, but unfortunately he was serving a life sentence in a Thai prison, so there was no way I could obtain a sample to test directly from him. Alphabiolabs suggested a grandparent test.”

When he eventually managed to locate his potential grandmother, the test showed only 13.5% probability that she was related to him. “I was absolutely devastated, but Alphabiolabs were extremely supportive and suggested I tried to find the potential grandfather to strengthen the results.” At long last, he found the man and test was done between all three people. “To my utter amazement, my report showed that there was a 99.93% chance of those people being my grandparents. I had no family at all prior to these results and it has opened doors of my life I didn’t know existed.”

Sometimes inconclusive results come from plain old trickery, says Thomas. “Particularly with the paternity tests – people try all sorts. For instance, we’ve had women who are so worried about the paternity test of their child coming back negative that they secretly swap the man’s swab with their own – assuming that it means the clear DNA link will make the result an instant positive. But sample testing can identify different gender chromosomes.”

Men try their luck, too. “We had one premiership footballer, who was was ordered to take a DNA test for legal purposes to determine whether he was the biological father of the child with a woman who was not his partner,” says Thomas. “In an attempt not to be found out, he asked his friend to take the test instead. His friend was at the arranged location for the sample collector to take a specimen, claiming to be the footballer. But again, our various strategies meant he didn’t succeed.”

Legal cases like this are just as common, if not more, than peace-of-mind tests at Alphabiolabs, says Thomas. “They are mainly paternity tests, the results of which are used for legal proceedings such as Child Support Agency cases. We do maternity tests too, especially for immigration cases.”

Fiona Uwera did one just this week. “My son wants to come to university here and the authorities said they only way this could happen is for me to show them our birth certificates or through a DNA test. In Rwanda, where we are from, I’ve never seen a birth certificate, so we opted for the test.”

For obvious reasons, the legal cases mean there has to be a witness present, which bumps up the cost significantly. “The individual can either do it at the GP surgery or one of our city centre drop-in centres,” says Thomas, who adds that some people do these tests, as well as peace-of-mind ones, at Alphabiolabs itself. Indeed, a few hang around anxiously in the foyer when I visit, clutching wads of £10 notes and testing boxes.

“On bad days, you get people shouting at you,” admits Hodgkinson. “One lady rang this week, whose son and daughter had a DNA test that revealed they weren’t full siblings – it proved he had a different dad. The woman was furious, believing she should have been consulted about the test. But normally people come round once we explain the process and she wound up apologising.”

Many cry. “We give people as much time as they need, regardless of whether they decide to go ahead with the test, and often provide a link to a Youtube video that psychologist Lisa Marlsand put together for us to help people work out if they’re ready.”

Samples of DNA being processed in the laboratory. Photograph: Christopher Thomond

It took Patricia Munn months of counselling to decide. “One night last September, my mother was due to die from a long illness and that’s when she told me that I wasn’t my dad’s,” says Patricia, 58. “In fact, she didn’t die for another two months, which meant she had to have some difficult conversations with me, in which it transpired that she’d had an affair with another man who we had known for 10 years. While my oldest three siblings were my dad’s, me and my younger brother were the other man’s, she said.

“It has been a difficult journey to reach this point where I feel I can do a test with one of my older siblings so I can finally find out the truth.”

Patricia believes her mother might be wrong. “She had a way of assuming something to be true and telling it as fact. I’m not sure what to expect.”

Again, we agree to talk later – and, lo and behold, the result shows that they are full siblings. “I have to admit I’m relieved,” she says. “I can get on with my life now.”

Death-bed confessions that prompt DNA tests are common, says Thomas, as are after-death DNA tests, which usually focus on probate and involve someone claiming to be a relative of the deceased. But the most recent area of work for Alphabiolabs is prenatal paternity testing. Until recently, says Thomas, this was only possible invasively. “Today, my test showed my husband to be the father, just as I knew it would,” says Nina Smith, 30. “We’d been together 16 years and I’ve never strayed. But completely out of the blue, he said he didn’t believe this baby was his and walked out on me. It was an awful shock. But he is, and has remained since our separation, a great dad to our daughter, so I’m hoping this test result will lead to him being just as good with this baby, even though he’d never get me back now.”

The competition for DNA testing is brutal, says Thomas. He chooses the word carefully. “Healthy competition is fine, but this industry isn’t regulated, which means anyone can set up a DNA testing service from their spare room, and frequently do. Either they send off the results to a testing lab in the US that may or may not be legitimate, or there have been cases of people just making up the results.”

Indeed, Simon Mullane was found in 2009 to have never carried out the tests of samples sent to his Dorset-based firm, High Profile. Charging clients £600 each, he just sent them all back as positive. “He was convicted of deception. We always say if you’re going to do a DNA test, use a Ministry of Justice-accredited lab.”

Thomas remembers one case where a woman whose DNA test from another company showed that her husband wasn’t the father of their child. “Their marriage fell apart and the man went off to join the army, going off to Afghanistan. She remained adamant, however, and eventually tracked him down and got us to do a test, which showed he was indeed the dad. By then, though, he’d lost four years of his child’s life, as well as his marriage.”

At 5.30pm, the lab staff are ready to go home, although not before the last handful of test results go out, one of which is Sue Mitchell’s. She was right – her sister is not a full sibling. “As much as I had my suspicions and thoughts about this family friend, I’m shocked and upset,” says Sue, her voice shaking. “I can’t believe it, really, and I’m not sure yet what I feel. But I do know my life will never be the same again.”